Ship Mountains Megabreccia: Implications for Miocene Extensional Tectonics, Eastern Mojave Desert, California

Abstract

The megabreccia of the Ship Mountains is part of a sedimentary sequence displaying characteristics of a catastrophic landslide deposit: (1) fractured clasts; (2) a graded, fine-grained upper mantling layer; (3) sandstone dikes injected from below; (4) fine-grained, massive matrix; and (5) local, monolithologic character. Its presence downslope from the headwall of a large Miocene normal fault, and correlation of breccia clasts with basement lithologies in the headwall area, further support the landslide hypothesis. This interpretation has important implications for the structural evolution of the area, suggesting two major phases of fault block rotation associated with extensional stresses along the western flank of the Old Woman Mountains structural horst.